The emission of CO2 from power plants has been identified as a factor potentially resulting in long-term environmental problems. Consequently, separation of CO2 from gaseous streams utilizing adsorption of gases and vapors by microporous solids has attracted attention, because of its great practical importance in the fields of gas separation and gas purification. Certain technologies based on the CO2 adsorption/desorption by using natural and synthetic zeolites are among the most effective methods.
Generally, two methods are used for CO2 adsorption/desorption utilizing zeolite adsorbents for CO2 separation: Temperature Swing Adsorption (TSA) and Pressure Swing Adsorption (PSA). In each of these techniques, a bed of zeolite adsorbent is exposed to a flow of feed air under temperature and pressure conditions allowing the zeolite adsorbent to adsorb some portion of the carbon dioxide and water vapor present in the feed air. This exposure continues for a period of time, and typically ceases prior to break-through of carbon dioxide and water in the exiting treated feed air. Following this exposure, the flow of feed air is ceased and the adsorbent is regenerated. In a temperature swing adsorber, the carbon dioxide and water are driven off from the adsorbent by heating the adsorbent in the regeneration phase. In a pressure swing adsorber, the pressure of the regeneration gas is lower than that of the feed gas and the change in pressure is used to remove the carbon dioxide and water from the adsorbent.
There has been extensive research on the equilibrium of carbon dioxide on zeolite adsorbent materials at ambient temperature and atmospheric pressure. It is established that zeolites efficiently remove carbon dioxide and water vapor from air streams at low temperatures, i.e., temperatures of about 40° C. or lower, because it more strongly adsorbs these components than it adsorbs nitrogen, oxygen or argon. However, at lower pressures, the carbon dioxide adsorption capacity of zeolites generally diminish rapidly as the temperature of the gas being separated increases, and the separation process can become infeasible at temperatures above about 40° C. This temperature limitation is further complicated by the heat of adsorption associated with zeolites, generating the tendency for adsorption bed temperatures to increase considerably, and often necessitating adsorption bed cooling with external refrigeration to maintain the gas at temperatures below 40° C. This places significant limitation on the application of zeolites for adsorption of carbon dioxide at high temperature, which is becoming increasingly significant in fields such as emission control of fossil-fueled power systems, natural gas treatments, purification of hydrocarbons, and production of hydrogen gas, among others. In these applications and others, it would be advantageous to provide a cycle where zeolite adsorbents could be utilized in a manner preserving the adsorption capacity at higher temperatures, such that parasitic external refrigeration loads could be reduced or eliminated. Such a capability could have particular import for gaseous stream separation in the production of H2 from shifted syngas using a water-gas-shift reactor, where any requirement to maintain temperature of the adsorbent bed below 20° C. levies significant refrigeration requirements. Such a capability has further advantage when separation operations occur prior to combustion of a non-adsorbed component, so that refrigeration requirements have a direct impact on the subsequent efficiency of the combustion. For example, the separation of CO2 and H2 from a water-gas shift (WGS) reactor in preparation for H2 combustion in an Integrated Gasification Combined Cycle (IGCC) plant.
It is known that the adsorption of some zeolites increases as the pressure of the adsorbent bed in increased for some applications. See e.g., Siriwardane et al, “Adsorption of CO2, N2, and O2 on natural zeolites,” Energy & Fuels, Vol. 17 (2003), discussing the adsorption of certain zeolites at temperatures of 25° C. and pressures up to 300 psig. See also Siriwardane et al, “Adsorption of CO2 on Zeolites at Moderate Temperatures,” Energy & Fuels, Vol. 19 (2005), discussing the adsorption of certain zeolites at temperatures up to 120° C. and pressures up to 300 psig, and regeneration at 300° C. under an N2 atmosphere. Generally speaking, investigations of diverse zeolites have indicated that the capacity for CO2 adsorption is enhanced when the partial pressure of CO2 increases. See Bonenfant et al, “Advances in principal factors influencing carbon dioxide adsorption on zeolites,” Sci. Technol. Adv. Mater. 9 (2008). A swing-adsorption cycle for CO2 separation using zeolites that takes advantage of the propensity for increased adsorption as pressure increases would allow zeolite use at higher temperatures, further reducing or eliminating parasitic external refrigeration loads. Additionally, and significantly, such a swing-adsorption process would have great benefit if desorption could occur at the higher pressure and produce a higher pressure CO2 product stream, reducing compression burdens which might exist in sequestration and storage operations.
Further, in CO2 separation for the purpose of storage and sequestration, purity of the CO2 product stream is of paramount importance. Often, in swing-adsorption cycles using zeolites for CO2 separation, the higher temperature or lower pressure regeneration must be assisted by the use of an inert regeneration gas to strip the adsorbed carbon dioxide and water from the adsorbent. Typical approaches utilize N2, He, or some portion of the post-adsorption feed gas. See e.g., U.S. Pat. No. 5,968,234, issued to Midgett, issued Oct. 19, 1999; see also U.S. Pat. No. 5,855,650, issued to Kalbassi et al, issued Jan. 5, 1999, among others. The use of regeneration gas often increases the recovery of CO2 in zeolite based swing-adsorption processes, however the practice inherently dilutes the recovered CO2 stream. In applications where purity of the generated CO2 stream is desired, such as in CO2 sequestration and storage operations, the diluted stream must undergo further separation for removal of the regeneration gas, and further inefficiencies result. It would be advantageous to provide a swing-adsorption process for CO2 separation using zeolites where sorbent regeneration could occur in an atmosphere of CO2. The use of CO2 as sweep gas during regeneration would result in the generation of a pure stream of the gas without the need for further gas separations.
Accordingly, it is an object of this disclosure to provide a method of CO2 separation utilizing zeolite adsorbents in a manner that preserves adsorption capacities at higher temperatures, such that parasitic external refrigeration loads are reduced or eliminated.
Further, it is an object of this disclosure to provide a method of CO2 separation utilizing zeolite adsorbents that takes advantage of the propensity for increased adsorption under increased CO2 partial pressure and allows for higher pressure regeneration, such that a higher pressure CO2 product stream is produced and compression burdens which might exist in sequestration and storage operations are reduced or eliminated.
Further, it is an object of this disclosure to provide a method of CO2 separation utilizing zeolite adsorbents in a TSA process where sorbent regeneration occurs in an atmosphere of CO2, such that a pure stream of the CO2 is generated without the need for further gas separations.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.